1. Field of the Invention
The present invention relates to a nitride semiconductor laser element.
2. Description of Related Art
In recent years, it has become possible for a nitride semiconductor laser as a nitride semiconductor element to perform oscillation in a wide wavelength range from an ultraviolet region to green. Therefore, the nitride semiconductor laser is expected to be applied not only for a light source of optical disk systems but for a light source of a wide variety of devices. Especially, the nitride semiconductor laser for emitting a light in a range of a visible light region closer to a side of a long wavelength range than a violet-blue region is expected as a light source for displays such as a light source for projectors and a light source for televisions.
JP 2009-200437 A discloses a semiconductor laser. The semiconductor laser includes a substrate provided thereon with an n-type clad layer, a first optical guide layer, an active layer, a second optical guide layer, and a p-type clad layer, wherein the second optical guide layer includes an InGaN region of which In composition gradually decreases from a side of the active layer toward a side of the p-type clad layer.
However, the semiconductor laser having such structure as disclosed in JP 2009-200437 A may cause an increase in threshold current because of incomplete optical confinement into the active layer. Such semiconductor laser may also cause light leakage to an electron barrier layer that significantly absorbs light, resulting in worsening of laser characteristics.
An n-side semiconductor layer includes a composition graded layer and such structure may cause a phenomenon in which a voltage increases. This problem can be avoided by doping with impurity, while impurity absorbs laser light. As a result, laser characteristics are lowered.
A semiconductor light emitting element in JP 2001-65632 A discloses a structure in which a region with low band energy and a region with high band energy exist in a p-side barrier layer. However, the p-side barrier layer does not function as an optical guide layer because of its small total thickness of 10 nm, leading to poor optical confinement, and thus laser characteristics may deteriorate. As a result, the semiconductor light emitting element employing the structure may cause a decrease in internal quantum efficiency.
In case the nitride semiconductor laser element has a quantum well structure including a well layer in an In-containing nitride layer, the In content must be increased as an oscillation wavelength becomes longer. As a result, large strain occurs inside the semiconductor layer, thus causing a decrease in internal quantum efficiency due to worsening of crystallinity and an increase in piezoelectric polarization. Thinning of the well layer can restrain the above-mentioned problem. However, carrier confinement to the well layer becomes inferior and overflow of the carrier occurs, leading to a decrease in internal quantum efficiency.
In a guide layer constituting a part of a waveguide included in a laminated structural body of the laser element, carrier concentration in regions other than the well layer increases due to an increase of overflow of the carrier from the well layer, thus causing light absorption, non-radiative recombination (Shockley Read-Hall (SRH) recombination, non-radiative Auger recombination) or a radiative recombination not contributing to oscillation. Whereby, worsening of laser characteristics, such as the above-mentioned decrease in internal quantum efficiency occurs.
In order to enhance carrier confinement, it is possible to suggest a structure in which an energy gap of a barrier layer is increased and a structure in which the thickness of a barrier layer is more increased. However, in this case, there may arise a problem that the barrier layer itself causes an increase in an operating voltage.
It is necessary for a laser element including an active layer between an upper clad layer and a lower clad layer to ensure refractive index difference for the confinement of light in the waveguide, including the active layer. Specifically, a nitride semiconductor having a large Al composition ratio is used for the clad layer, thus causing a problem of crystallinity. This is because crystallinity of a nitride semiconductor containing Al tends to cause deterioration due to the occurrence of cracks as compared with the other nitride semiconductors that do not contain Al.